WOx Thin Films Research Articles

Simultaneous enhancement of visible light absorption and energy storage performances of W5+ implanted WO3-x thin film electrodes

Simultaneous enhancement of visible light absorption and energy storage performances of W5+ implanted WO3-x thin film electrodes

Introducing oxygen vacancies into WO3 thin film for improving hydrogen sensing performance of Pd/WO3-x/AAO sensors

Introducing oxygen vacancy into semiconducting metal oxides (SMOs) is a strategic approach to promote their gas sensing performances. In this paper, the WO3-x thin films were meticulously deposited onto an anodic aluminum oxide (AAO) substrate, and the oxygen vacancies concentrations of WO3-x thin films were changed by adjusting the ratio of flux of Ar to O2 in magnetron sputtering process. Sequentially, a layer of palladium (Pd) was deposited on the WO3-x/AAO films, creating a nano-mesh structured hydrogen sensor with Pd as the catalytic element. The resulted amorphous WO3-x samples with varying concentration of oxygen vacancies were thoroughly analyzed using XPS and ESR, which confirmed that the oxygen vacancies concentration escalated with an increase in the ratio of flux of Ar to O2. The hydrogen sensitivity of prepared Pd/WO3-x/AAO sensors were examined. Our findings revealed an enhancement in hydrogen sensitivity for the sensors with an optimal concentration of oxygen vacancies. However, an excess of these vacancies was found to deteriorate their hydrogen sensing performance. Notably, the optimized Pd/WO3-x/AAO sensor exhibited a marked response even at a hydrogen concentration as low as 100 ppb, demonstrating its high sensitivity and selectivity towards hydrogen detection.

Controllable physicochemical properties of WOx thin films grown under glancing angle.

In this work, various physicochemical properties are investigated in nanostructured WOx thin films prepared by radio-frequency magnetron sputtering for optoelectronic applications. A glancing angle of 87° is employed to grow films of different thicknesses, which are then exposed to post-growth annealing. Detailed local probe analyses in terms of morphology and work function of WOx films are carried out to investigate thickness-dependent property modulations of the as-deposited and annealed films. The analyses show a reasonably good correlation with photoelectron spectroscopic measurements on the films and the bulk I-V characteristics acquired on a series of WOx/p-Si heterojunction diodes. The presence of a critical WOx thickness is identified to regulate the rectification ratio values at the WOx/p-Si heterostructures and increase in series resistance within the bulk of the films. The present study provides valuable insights to correlate optical, electrical, and structural properties of WOx thin films, which will be beneficial for fabricating WOx-based optoelectronic devices, including photovoltaic cells.

Characterization of photocatalytic hybrid TiO2–WOX thin films deposited via co-sputtering

Photocatalytic hybrids of titanium dioxide (TiO2) and tungsten oxide (WOX) can exhibit high photocatalytic activity under weak ultraviolet (UV) and visible-light irradiation. In this study, hybrid TiO2–WOX thin films deposited via co-sputtering are characterized using radiofrequency sputtering at various compositions, and their photocatalytic activity in the degradation of a methylene blue solution is evaluated. X-ray photoelectron spectroscopy measurements show enhanced W(4f) and decreased Ti(2p) signal intensities with increasing WOX content. Further, chemical shifts toward lower binding energies are observed as the compositions of TiO2 and WOX become equivalent. X-ray diffraction analysis reveal decreased anatase TiO2 peaks and monoclinic WOX peaks with the formation of amorphous structures as the compositions of TiO2 and WOX become equivalent. Atomic force microscopy images show homogeneous and flat structures on the surfaces of the hybrid TiO2–WOX thin films. The deposition rate decreases with decreasing WOX sputter power, whereas the optical bandgap decreases with increasing WO3 content. Furthermore, the photocatalytic activity under UV irradiation improves with increasing TiO2 content, whereas that under visible-light irradiation improves with increasing WOX content. This approach can provide the guidelines to fabricate the high-performance photocatalytic materials that can be used in different condensed thin-film devices.

The effect of DC power and annealing on the structural, optical, and electrical characteristics of sputtered amorphous WOX thin films

Tungsten oxide (WOX) is considered as one of the promising materials due to its unique properties in widespread optoelectronic and photonic applications. This study reports on the deposition of WOX thin films via reactive magnetron sputtering technique with various low DC power and constant sputtering pressure at room temperature. XRD results reveal that an amorphous state for all the as-deposited films, while annealed films at 650 °C have WO3 monoclinic structure. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) studies indicate all as-deposited films have a dense, homogeneous, and quite smooth surface with a root-mean-square roughness (σRMS) below 1 nm. Annealing significantly affects film morphology, the formation of large grains and increasing the surface roughness. Optical parameters were examined by ultraviolet–visible (UV–Vis) spectrophotometer. Increasing sputtering power leads to a decrease in transparency value, an increase in absorbance, a reduction in penetration depth, a red shift of the fundamental absorption edge, and a reduction in the band gap energy from 3.06 eV to 2.61 eV. The annealed films exhibit a smaller optical band gap compared to as-deposited films in the range of 2.44 eV–2.93 eV. The temperature dependence of the sheet resistance corresponds to the Arrhenius law for as-deposited films with DC power of 10 W–30 W, indicating semiconducting nature. A decreasing trend in activation energy (0.283–0.055 eV) was observed with increasing DC power. All the annealed films exhibit insulator nature with a resistance of more than 100 MΩ. Analysis of these results suggests that the desired structural, optical, and electrical properties can be achieved by controlling deposition conditions.

Hot wire Oxidation–Sublimation derived work function tunable WOx thin films for building hole-selective contacts

The replacement of doped amorphous silicon (a-Si) with transparent passivation materials is an alternative approach to promote the efficiency of silicon heterojunction (SHJ) solar cells. Transition metal oxide, especially tungsten oxide (WOx) is attractive for adhibition in SHJ solar cells as the substitute, due to the wide-bandgap, adjustable work functions, and relatively high stability. Here we introduced a hot wire oxidation-sublimation deposition (HWOSD) for preparation of WOx films, realizing precise modulation of their work function ranging from 4.9 to 5.4 eV. Notably, no loss of passivation quality relevant to the pre-deposited passivating layer of hydrogenated amorphous silicon (a-Si:H) was betrayed when completed the coating of WOx films. The hole-selective contacts of a-Si:H/WOx stack are utilized in SHJ solar cells, delivering a power conversion efficiency of 21.0 %.

Fine-tuned photochromic WO3-x thin films: A detailed study from structural analysis to UV photo-sensing application

The oxygen substoichiometry of as-prepared WO3 (tungsten trioxide) nano-powders (NPs) from a polyol process was tuned via peptization of the NPs in aqueous solutions of different Cr2O72− oxidizing concentrations. The as-synthesized materials have been characterized by X-ray scattering (XRD and PDF), transmission electron microscopy (TEM), X-ray photoelectron (XPS), and UV-VIS photochromic activity. The local atomic structure of WO3-X was investigated using total scattering atomic pair distribution function (PDF) analysis based on X-ray total scattering data collected on powder at ambient conditions. The PDF analysis confirms that the crystal structure of all studied samples can be described in terms of very small crystallites with a P21/n space-group monoclinic framework but with anomalous unit cell distortion parameters, indicating that small crystallite sizes resulted in a larger monoclinic distortion (as measured by the beta angle of the unit cell). The nanometer dimension of the crystallites combined as well as the oxygen-tungsten stoichiometric ratio control, are key features for optimized photochromic properties. Moreover, we present the fabrication of a WO3-x thin film based UV photosensor, which was carried out on silica glass substrates via the dip-coating method. The obtained films exhibited a UV photoresponse and photoelectric characteristics at 5 V bias voltages able to detect very low UV doses, inferior to 10 W/m2. The photo-detection measurements prove the usability of our device as a UV photodetector with a good responsivity of 0.37 A/W and external quantum efficiency of more than 100% even at a very low power density (9.2 W/m2) of UV illumination.

High porosity and oxygen vacancy enriched WO3-x thin films for room temperature hydrogen gas sensors

Due to the increase in demand for hydrogen in several fields, sensors that detect the presence of hydrogen gas more precisely even at room temperature (30 °C) are required for its safe use. In this work, we report the hydrogen gas sensing performance of the WO3-x thin film deposited at different oxygen partial pressures (10%, 15%, and 20%) onto SiO2/Si substrate using a radio frequency (RF) magnetron sputtering technique. To study the hydrogen gas sensing performance, Ag/WO3-x/Ag test-device is fabricated by depositing interdigitated Ag electrodes onto the WO3-x gas sensing layer via thermal evaporation. The performance of the obtained sensors is investigated at different hydrogen concentrations (100 ppm–300 ppm) and different operating temperatures (30°C–300 °C). WO3-x thin films deposited at 10% of pO2 show a maximum response of 3.40% for 100 ppm of H2 gas even at room temperature, which is due to the high crystallinity, high porosity, and the presence of more oxygen vacancies. The oxygen vacancies that are created during the reactive magnetron sputtering would enhance the hydrogen adsorption and hence superior hydrogen sensing results achieved. Herein, the response and recovery times are measured in most of the sensing conditions and the sensing mechanism has been discussed.

Enhancing the NIR shielding ability and controlling the surface wettability of RF magnetron sputter deposited WO3-x thin films by tuning the oxygen vacancies

Enhancement in the near-infrared (NIR) shielding property of tungsten oxide (WO3-x, 0 < x < 1) thin films by the induction of further oxygen vacancies via vacuum annealing is discussed in this work. The property is comprehensively analysed by UV–Vis–NIR spectroscopy and variable angle spectroscopic ellipsometry techniques. From transmittance spectra, an average visible transmittance of ∼48% along with an average NIR transmittance of ∼15% (with Tvis/Tsol ratio of 1.49) is obtained for WO3-x thin films vacuum annealed at 500 °C for 1.5 h. The pronounced NIR absorption is attributed to the enhanced polaron absorption and surface plasmon resonance effects produced by the increased oxygen vacancies in the film. In order to realize their potential as energy saving window coatings, thermal shielding test is performed in which vacuum annealed WO3-x thin films could restraint the indoor temperature of the heat box efficiently. X-ray diffractometry, field emission scanning electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy measurements were also performed on the films for elucidating structural, morphological, and compositional characteristics. In addition, contact angle measurements revealed that vacuum annealing could reduce the surface wettability of WO3-x thin films considerably. The present work is significant in the field of development of NIR shielding materials for energy conserving smart window applications, especially for buildings and automobiles.

Deposition of tungsten in methane atmosphere using a low energy plasma focus device

Deposition of tungsten in methane atmosphere was performed using a low energy plasma focus device. The resulting compounds were synthesized and investigated during the deposition process. Carbon-rich contamination was identified as an organic viscous liquid crystal defect. Tungsten carbide (W2C) and dark-black oxygen-deficient tungsten trioxide (WO3-x) thin films were synthesized by plasma focus device using different number of focused shots. The synthesis of each thin film, which was dependent upon operation of plasma focus device was verified and discussed. Tungsten carbide W2C was synthesized during the deposition process while the dark-black WO3-x was created after the deposition process in atmosphere. All compounds were investigated by spectroscopy, crystalline, and microscopy methods. Nano indentation test disclosed higher value of hardness for the film containing tungsten carbide compound (10.01 GPa) compared to both pure tungsten (5.72 GPa) and the dark-black WO3-x thin film (6.83 GPa). The verified band gap energy of dark-black WO3-x was around 1.475 eV.

Photovoltaic and electrical investigation of In/WOx/CuPc/In heterojunctions with light intensity-dependent NDR behaviours

A peripherally tetrachalcone substituted copper(II) phthalocyanine was achieved by cyclotetramerization of the phthalonitrile in the presence of anhydrous CuCl2. The modified CuPc was thermally coated on WOx thin films from which a In/WOx/CuPc/In heterojunction was obtained. The surface of the thin films was investigated using scanning electron microscopy (SEM), where energy dispersive X-ray spectra (EDX) was used to confirm chemical structure. The crystal structures of the thin films were characterized using X-ray diffractometry (XRD). To determine the photovoltaic properties of the heterojunction, I–V plots were obtained under dark and various levels of illumination. It was seen that the heterojunction was responsive to the external light. Using the I–V plots, different diode parameters such as ideality factor, barrier height, series resistance, etc., were calculated. These plots also revealed that the In/WOx/CuPc/In heterojunction exhibits negative differential resistance (NDR) characteristics. Photodiode characteristics indicate that the heterojunction exhibits self-powered photodiode behaviours. It was seen that In/WOx/CuPc/In heterojunctions are sensitive to the external light that photovoltaic characteristics alter when the light intensity alters. However, no clear relation between the light intensity and photovoltaic characteristics were evidenced.

Substrate Effects on Growth Dynamics of WTe2 Thin films

AbstractSynthesis of transition metal dichalcogenides (TMDCs) has been achieved through the direct conversion of metal and metal‐oxide films, demonstrating the ability to grow large area thin films with uniform thickness on a variety of substrates and direct control over the growth orientation (horizontal vs vertical) of the TMDC layers. However, the synthesized TMDC films often exhibit small grains and are more defective than their bulk counterparts. This is especially true for 2D telluride films due to the low reactivity between tellurium and transition metals such as W and Mo. In this work, the substrate interactions is examined for WTe2 converted from amorphous WOx thin films grown by atomic layer deposition, on c‐plane sapphire and SiO2 through tellurization at high temperatures. Similar to TMDC telluride MoTe2, the formation of monolayer WTe2 on sapphire is observed, but not on SiO2. However, due to decreased diffusion of W on sapphire compared to Mo, the formation of WTe2 flakes instead of continuous films is observed, providing insight into the role of the specific transition metal during the direct synthesis of TMDC telluride films.

Investigation of Morphological, Optical, and Dielectric Properties of RF Sputtered WOx Thin Films for Optoelectronic Applications.

Tungsten oxide (WOx) thin films were synthesized through the RF magnetron sputtering method by varying the sputtering power from 30 W to 80 W. Different investigations have been conducted to evaluate the variation in different morphological, optical, and dielectric properties with the sputtering power and prove the possibility of using WOx in optoelectronic applications. An Energy Dispersive X-ray (EDX), stylus profilometer, and atomic force microscope (AFM) have been used to investigate the dependency of morphological properties on sputtering power. Transmittance, absorbance, and reflectance of the films, investigated by Ultraviolet-Visible (UV-Vis) spectroscopy, have allowed for further determination of some necessary parameters, such as absorption coefficient, penetration depth, optical band energy gap, refractive index, extinction coefficient, dielectric parameters, a few types of loss parameters, etc. Variations in these parameters with the incident light spectrum have been closely analyzed. Some important parameters such as transmittance (above 80%), optical band energy gap (~3.7 eV), and refractive index (~2) ensure that as-grown WOx films can be used in some optoelectronic applications, mainly in photovoltaic research. Furthermore, strong dependencies of all evaluated parameters on the sputtering power were found, which are to be of great use for developing the films with the required properties.

Electrical and optical properties of the WOx thin films, prepared by magnetron sputtering, and analysis of their highest value

In this article, optical and electrical properties of tungsten oxide thin films, prepared by magnetron sputtering in the atmosphere of various oxygen content, were analysed. Tungsten oxide is a material, which is widely used in modern applications such as smart windows, anti-steam mirrors, and gas sensors. Five sets of tungsten oxide thin films were deposited by magnetron sputtering in the mixed argon-oxygen atmosphere composed of various content of reactive gas, i.e., from 5% to 15%. In each case, other deposition process parameters were the same. The thickness of the thin films was ca. 160 nm and it was measured with the optical profilometer. It was noticed that along with the increase in the proportion of reactive gas in the process, thin film samples changed their colour from metallic, through navy blue to blue and simultaneously they became increasingly transparent. Measurements of electrical properties, made using the four-point probe and optical properties, performed with the aid of a spectrophotometer showed that increasing the proportion of oxygen in the process increased the average transmission in the visible wavelength range and the resistivity of the prepared WOx thin films. In the article, the Figure of Merit (FoM) coefficients are also presented, which determine the quality of the thin film samples. It was shown that the highest value of the FoM was observed for WOx thin film characterised by a relatively high average transmission in the visible wavelength range and low value of resistivity. The performed measurements showed that the gas atmosphere during magnetron sputtering process leads to the tailoring of the optical and electrical properties of tungsten oxide thin films, which in turn makes it possible to design and apply such thin film coatings in transparent electronics. Keywords: magnetron sputtering, thin films, tungsten oxide, optical and electrical properties

Effects of substrate temperature and sputtering gas composition on physical properties and photocatalytic activities of WOx thin films deposited via radiofrequency sputtering

Tungsten oxide (WOx) is expected to act as a photocatalytic material under visible light. We have deposited WOx thin films using radiofrequency sputtering and evaluated the photocatalytic activities of the films via degradation of methylene blue solution. The optical absorbance, crystallization, and surface morphology of the WOx thin films were also investigated. The absorption edge of the WOx thin films was shifted to the long-wavelength region when the substrate temperature was high and the O2 gas flow rate was low. Crystallization proceeded when the substrate temperature was high and additional WOx diffraction peaks were appearing with low O2 gas flow during growth. Furthermore, the grain size of the WOx thin films was smaller when the O2 gas flow was high. The photocatalytic activity was higher when the substrate temperature and O2 gas flow rate were low.

Effect of substrate temperature on the near-infrared shielding properties of WO3-x thin films deposited by RF magnetron sputtering

Near-infrared (NIR) shielding materials emerge as potential candidates for energy conservation applications. In this work, tungsten oxide (WO3-x, 0 < x < 1) thin films were deposited on soda-lime glass substrate by RF magnetron sputtering at various substrate temperatures and NIR shielding properties of the films were studied. Structural, morphological and compositional properties of the films were also investigated. Optical constants, thickness and roughness of the films were extracted from spectroscopic ellipsometric (SE) measurements. The correlativity of SE with UV–Vis–NIR spectrophotometry, cross-sectional field emission scanning electron microscopy and atomic force microscopy was investigated. Of all the films, WO3-x thin films deposited at 500 °C showed the lowest NIR transmittance of about 23% with the highest visible transmittance of above 70% corresponding to a Tvis/Tsol ratio of 1.35 and NIR shielding value of 75%. WO3-x coated soda lime glass shows better NIR shielding performance compared to commercial soda lime glass in the thermal shielding test. The underlying reason for NIR absorption is the presence of oxygen vacancies which promote formation of W5+ ions and the resultant combined effects of polaron absorption and surface plasmon resonance. This work is significant in utilizing a facile technique for developing undoped WO3-x thin films for efficient NIR shielding applications.

Sputtered WOx thin film as the electron transport layer for efficient perovskite solar cells

Sputtered WOx thin film as the electron transport layer for efficient perovskite solar cells

Microstructural, mechanical and optical properties of tungsten oxide coatings fabricated using superimposed HiPIMS-MF systems

In this study, the under-stoichiometric WOx thin films were fabricated using a high power impulse magnetron sputtering (HiPIMS) and a superimposed HiPIMS-mid-frequency (MF) system. The oxygen flow rate ratios were varied from 33% to 83% while maintaining the same working pressure for all depositions at 0.4 Pa. The ratio of oxygen to tungsten, x value of WOx, increased from 2.07 to 2.40 as the oxygen flow rate ratio increased from 33% to 83%. For the WOx thin films deposited using oxygen flow rate ratios higher than 33%, the nanocrystalline monoclinic W25O73 phase was produced by a superimposed HiPIMS-MF or by a pure HiPIMS. As compared with the pure HiPIMS, the superimposed HiPIMS-MF technique fabricated WOx thin film with higher deposition rate and higher hardness. The WO2.28 film showed the highest hardness of 8.6 GPa. The highest optical bandgap value of 3.38 eV was achieved in WO2.29 film. The oxygen content, the O/W ratio, and the average transmittance of thin films increased with increasing O2 flow rate ratio.

Tuning the Electrical Properties of Tungsten Oxide Thin Films Deposited by Reactive Magnetron Sputtering

Tungsten oxide films are deposited onto glass and silicon substrates using reactive magnetron sputtering. Several studies have revealed difficulties in studying the electrical properties of resistive WOx films. The main objective of this work is to propose the capacitance meter as a method for studying the electrical properties of resistive WOx films. In addition, we aim to establish the correlation between the deposition process and WOx physico-chemical properties. The study of the W-Ar-O2 system hysteresis permitted us to gather the films into four zones, which were delimited by different oxygen flow rate intervals. The identification of these zones was confirmed by the deposition rate, target voltage, chemical composition and electrical properties of the films. A gradual evolution of the capacitance-voltage curves of the metal-oxide-semiconductor structures, with the WOx thin films as the oxide layer, was globally observed with increasing oxygen flow rate. Ion density (Nss) and flat band voltage (Vfb) evolved inversely to oxygen flow rate. The relationship between the evolution of Nss and Vfb and the increase in oxygen flow rate reflects the improvement in oxygen stoichiometry in the WOx films. The WOx-Si interface trap density distribution (Dit) was also studied using the Terman method. It was observed that the films closest to stoichiometry, i.e., WO2 or WO3, showed the lowest values of Dit and Nss.

Patterning of WOx, VOx, and MoOx thin-films with picosecond and nanosecond laser sources

Transition metal oxide (TMOs) layers have interesting properties as selective contacts, i.e., hole or electron transport layers for novel semiconductor devices. Especially, oxides of molybdenum (MoO3), vanadium (V2O5), and tungsten (WO3) show good bahaviour acting as front hole-selective contacts for n-type crystalline-silicon heterojunction solar cells. Laser scribing has been widely used for thin-film ablation and seems the appropriate technology for device manufacturing with such non-conventional materials. In this work, we study the laser scribing of non-stoichiometric evaporated WOx, VOx, and MoOx films with three different wavelengths (1064, 532, and 355 nm) with pulse duration in the nanosecond and picosecond regimes. The selection of the proper laser source allows a wide parametric window, with complete removal of the TMO films and no alteration of the silicon substrate. The results on the isolation of diodes and their electrical characteristics show the quality of the laser scribing processes.